1. Field of the Invention
The present invention relates to an inspection method for an image reading apparatus, and more particularly, to an inspection apparatus for performing an inspection and an evaluation for color misregistration in a reading apparatus for reading a color image.
2. Description of the Related Art
Conventionally, in a flatbed image reading apparatus (image scanner) for reading image information on a surface of an original, in order to simplify the structure of the apparatus, there has been of an integrated-carriage scanning type in which a reflection mirror, an imaging optical system, a line sensor, and the like are integrated into one unit to scan a surface of an original.
In a conventional image reading apparatus of the integrated-carriage scanning type, a light beam emitted from an illumination light source illuminates an original placed on an original table. An optical path of the light beam is deflected when the light beam reflected on the original is reflected in the carriage by means of multiple reflection mirrors. The light beam is imaged on a surface of a line sensor through an imaging optical system. Then, the carriage is moved by a sub-scanning motor in a sub-scanning direction with respect to the surface of the original to read image information of the original. The line sensor has such a configuration that multiple light receiving elements are arranged in a main scanning direction.
FIG. 9 is an explanatory diagram of a basic configuration of an image reading optical system. Line sensors 85R, 85G, and 85B for reading respective colors R (red), G (green), and B (blue) read, via an imaging optical system 84, pieces of image information of reading ranges 87R, 87G, and 87B on the surface of the original, respectively. The carriage moves relative to the surface of the original, and hence the image information on the same point on the surface of the original may be read by different color light beams after given time intervals. In a case where the imaging optical system 84 is formed of a coaxial lens system having a rotationally symmetric surface, longitudinal chromatic aberration and lateral chromatic aberration occur due to characteristics of a lens material. Hence, compared with the line sensor 85G serving as a reference, defocusing or position displacement occur in line images formed on the line sensors 85B and 85R. Consequently, when images of the respective colors are superimposed, the resultant image is such an image that color blurring or color misregistration is conspicuous.
In an imaging optical system using a coaxial lens system having a rotationally asymmetric surface, in which an anamorphic surface is disposed on an image plane side of the imaging optical system (Japanese Patent Application Laid-Open No. 2000-171705), distortion in the sub-scanning direction is likely to occur due to a manufacturing error of the rotationally asymmetric surface, in addition to chromatic aberration due to characteristics of a lens material. Japanese Patent Application Laid-Open No. 2002-335375 discloses a so-called off-axial optical system. That is, in a non-coaxial optical system, a concept of a reference axis is introduced, and a constituent surface is made into an asymmetrical aspherical surface, to thereby correct aberration. However, if the constituent surface has a manufacturing error or a position error, distortion occurs due to focus position displacement on the image plane in the main scanning direction (in the main scanning cross-section) or in the sub-scanning direction (in sub-scanning cross-section), which leads to reduction in focal depth and magnification deviation. Consequently, a read line is curved. In particular, distortion in the sub-scanning direction is very likely to occur. When image information is read by line sensors of the colors R, G, and B, as illustrated in FIG. 10, imaging positions on the line sensors located on outer sides of the sub-scanning direction are displaced in the sub-scanning direction due to this distortion, increasing from the optical axis toward both ends of the line sensors. As a result, color misregistration in the sub-scanning direction occurs in a superimposed image.
In order to measure such a color misregistration amount, the following inspection apparatus is proposed. That is, a test chart in which white bars and black bars are alternately printed in the sub-scanning direction is placed on an original surface of a main body of the apparatus, and the test chart is read, thereby performing a calculation and an inspection for a color misregistration amount (see Japanese Patent Application Laid-Open No. 2000-332958).
Further, in order to calculate a color misregistration amount based on a distortion amount, the following method is proposed. That is, at a position equivalent to a position of a surface of an original, there is provided a chart for detecting reading-position displacement in sub-scanning direction, which includes a triangular chart or a chart having different shapes between the main scanning direction and the sub-scanning direction, and the chart is read to obtain a signal, which is used for calculating a distortion amount, thereby calculating a color misregistration amount (see Japanese Patent Application Laid-Open No. 2008-078877).
Japanese Patent Application Laid-Open No. 2000-332958 has the following problem. That is, a reading unit, which is mounted with an imaging optical system, needs to be moved in the sub-scanning direction (hereinafter, referred to as scan operation) so as to detect a color misregistration amount in the sub-scanning direction, and hence the imaging optical system alone may not be inspected, meaning that an inspection may not be performed until the final process of the product. Further, instead of the reading unit, by causing the test chart to perform the scan operation, the imaging optical system alone may be inspected, but there are problems that the inspection apparatus becomes upsized and complicated, and that an inefficient inspection time occurs due to the scan operation.
In Japanese Patent Application Laid-Open No. 2008-078877, a distortion amount is calculated by using the chart for detecting reading-position displacement in sub-scanning direction, which includes a triangular chart or a chart having different shapes between the main scanning direction and the sub-scanning direction. However, the distortion amount is calculated based on a single chart, and hence Japanese Patent Application Laid-Open No. 2008-078877 has a problem in measurement accuracy because of the small number of samplings.